This invention relates to an electronic switched-mode power supply and the use of such a switched-mode power supply.
From DE-41 22 544 C1 an electronic switched-mode power supply of this type is known, being illustrated in simplified form in FIG. 1 and explained in more detail in the following. This electronic switched-mode power supply comprises a primary switched-mode flyback converter having a transformer 1, a transistor 2, and a diode 3 provided in the load circuit. The flyback converter is energized via a bridge rectifier arrangement 4 from a DC or AC supply whose voltage may be in the range of between 100 and 250 volts or also 12 or 24 volts, and whose frequency may be nearly arbitrary where an AC supply is used. The output voltage is applied to the input of the flyback converter or the electronic control and regulating circuitry through a filtering and smoothing arrangement 5 represented in this Figure only as a capacitor for the sake of simplicity.
Such a flyback converter may be used, for example, in a small appliance such as a shaving apparatus. This shaving apparatus may then be connected to supplies having different voltage values depending on the country. It is desirable in this case to be able to supply such a shaver with energy during camping trips or on board of a boat from the boat's electrical system. The voltages applied may then be 12 V DC or 24 V DC.
Connected in parallel with the DC terminals is the series arrangement comprising the primary winding 6 of the transformer 1 and the collector-emitter circuit of the transistor 2, and a capacitor 7. Connected to the base of the transistor 2 is a resistor 8 which is coupled to the positive pole of the input voltage source. In addition, the base of the transistor 2 is connected to the negative pole of the input voltage source via the collector-emitter circuit of a further transistor 9. The emitter of the transistor 2 is connected to the cathode of a Zener diode 10 having its anode connected to the base of the transistor 9 and also, through a further resistor 11, to the negative pole of the input voltage source. Moreover, the emitter of the transistor 2 is connected through a further resistor 12 to a first end of the secondary winding 13 of the transformer 1. The respective directions of winding of the primary 6 and secondary 13 of the transformer 1 are indicated by the dots shown in the Figure.
A feedback capacitor 14 is connected, through a feedback resistor 15, to the base of the transistor 2, its other side being connected to a second end of the secondary winding 13 of the transformer 1. Connected to the first end of the secondary winding 13 of the transformer 1 is the electrical energy-absorbing device. In the embodiment shown, the electrical energy-absorbing device is a storage battery 16. An electric motor, for example, which is not shown and which is energized by the storage battery by means of an On/Off switch, may be connected to this storage battery 16 when the electronic switched-mode power supply is not connected to a supply voltage. Equally, a diode 19 is provided which connects the first end of the secondary winding 13 of the transformer 1 to the junction of the feedback capacitor 14 and the feedback resistor 15.
To limit the back voltage, a circuit is provided in parallel with the primary winding 6 of the transformer 1, comprising the series arrangement of a Zener diode 17 and a further diode 18, the diodes having their respective cathodes connected to each other.
The mode of operation of the circuit arrangement of FIG. 1 may be explained in greater detail as follows. From the input voltage terminals, an initially low base current drives, through the resistor 8, the transistor 2 operating as a switching transistor. As a result of the transistor 2 turning on, a positive feedback effect occurs through the collector-emitter circuit of the transistor 2 and the primary winding 6 of the transformer 1, caused by the induced voltage resulting from the rising current in the primary winding 6 of the transformer 1. This positive feedback effect causes the transistor 2 to be driven additionally, rendering it conductive.
The collector current of the transistor 2 rises linearly. This produces a proportional voltage drop across the resistor 12. When, as a result of the rising collector current, the sum of the voltage across the resistor 12 and the voltage of the storage battery 16 reaches a value exceeding the breakdown voltage of the Zener diode 10, the transistor 9 goes into conduction.
This causes the base of the transistor 2 to be connected directly to the one terminal of the input voltage source, whereby the base current is withdrawn from the transistor 2. The transistor 2 thereby stops conducting, and current flow through the primary winding 6 of the transformer 1 is cut off abruptly.
This induces in the secondary winding 13 of the transformer 1 a voltage of a polarity that is opposite to the polarity during the interval when the transistor 2 is conducting. In respect of a current caused by this induced voltage, the diode 3 has a polarity in the forward direction. Current is thus supplied to the load (storage battery 16) during the inverse cycle until the energy stored in the transformer 1 is delivered to the load.
During the reversal process of the transformer 1, the diodes 17 and 18 connected in parallel with the primary winding 6 of the transformer 1 limit the back voltage peak.
As a result of the voltage induced across the secondary winding 13 of the transformer 1, the capacitor 14 received a positive charge on the side identified by "A" during the ON period of the transistor 2. Correspondingly, a negative charge was applied to the side of the capacitor 14 identified by "B". The diode 19 has a polarity in the reverse direction related to the voltage induced across the secondary winding 13 of the transformer 1 during the ON period of the transistor 2.
With the transistor 2 in its non-conductive state, a voltage is induced across the secondary winding 13 of the transformer 1 whose sign is precisely inverse to the voltage induced across the secondary winding 13 of the transformer 1 in the conductive state of the transistor 2.
As a result, the capacitor 14 reverses its charge during the OFF period of the transistor 2. Point B is connected to the positive voltage through the diode 19 whose polarity is in this case in the forward direction, while point A is connected to the negative voltage.
Accordingly, after the transformer 1 has delivered its stored energy, the capacitor 14 has a positive charge at point B, and correspondingly a negative charge at point A.
This voltage across the capacitor 14 supports the switching process in the subsequent switching of the transistor 2 to the conducting state, because the base-emitter current through the transistor 2 is supported by the sign of the voltage across the capacitor 14. In particular at low-level input voltages as, for example, a 12 V DC voltage, the turn-on process of the transistor 2 is supported by the sign of the voltage across the capacitor 14. Hence operation of the flyback converter is stabilized in particular at low-level input voltages.
During the subsequent conductive state of transistor 2, the capacitor is again charged as described above, receiving a positive charge at point A and a negative charge at point B.